[FEATURE] Support for locally referenced packages

[madpah]

Pip allows for packages to be referenced locally using relative and absolute file paths (as documented in Pip's Examples).

This feature will ensure that all valid variants of locally referenced packages can be parsed from requirements.txt files.

This Feature should also resolve the following reported issues:

• Failing requirements with local path #54
• Optional dependencies support #68
• Local file parse failure #18 ?